Voltage controlled oscillator (VCO) circuits are well known in the art and are utilized in a number of applications, for example, within a phase-locked loop. A VCO circuit typically includes a variable element such as a capacitor that may be varied to adjust the frequency of an output signal of the VCO circuit.
A VCO circuit typically includes some type of output stage such that the output impedance of the VCO circuit is a suitable value. An output stage that is coupled to an oscillator circuit via a coupling capacitor is disclosed in U.S. Pat. No. 5,107,228 and having an issue date of Apr. 21, 1992. This patent discloses a VCO circuit having an output amplifier for providing an adjustable output impedance. However, the circuits shown in FIGS. 1 and 2 have a common emitter output stage which does not provide high gain amplification. Further, the circuits shown in FIGS. 1 and 2 require a separate bias current for both the output amplifier and the oscillation stage and, thus, are not power efficient.
Hence, there exists a need to provide an improved VCO circuit having an output stage with an adjustable output impedance for operating at low supply voltages.